Water may be supplied to multi-unit structures or buildings such as hotels, apartment buildings, and the like by heating water in a tank so water at the tank outlet is at a desired temperature. The water circulates through a pipeline past the various units, and then back to the tank for recirculation. Older systems merely set the temperature of water at the tank outlet to a predetermined level such as 145.degree. F., which was sufficient to assure that all units received water at a sufficient temperature such as 110.degree. F. to avoid complaints. Considerable amounts of heat are lost along the pipeline extending between the tank outlet and the recirculating inlet, with the heat loss increasing with increasing water temperature in the pipeline. These losses are minimized by maintaining the temperature of water at the tank outlet, and therefore in the pipeline, at as low a level as possible, while still assuring that a minimum hot water temperature such as 110.degree. F. is available to every unit.
An earlier U.S. Pat. No. 4,522,333, owned by the assignee of the present application, describes an improved system where the temperature T.sub.1 at the water tank outlet is adjusted according to the anticipated demand for water, based on the history of water usage for that structure (e.g. hotel). For example, if the previous pattern of demand shows high demand at 7am on Wednesday, then the temperature T.sub.1 at the tank outlet may be brought up to 145.degree. F. shortly before 7am to assure adequate hot water. On the other hand, if the history shows a very low demand at 2am on Wednesday, the temperature T.sub.1 may be set to 115.degree. F., which will assure an adequate water temperature (e.g. 110.degree. F.) at even a last unit along the pipeline. A system for more closely controlling the water heater is described in another U.S. Pat. No. 4,620,667 owned by the assignee of the present application, which accounts for "stacking" of water in the water tank (cold water falling to the bottom of the tank), and which attempts to determine changes in heat loss along the pipeline by determining the amount of heat required to maintain the desired T.sub.1 when there is substantially no demand for water (such as at 2am).
While the systems described in the above-mentioned patents enable considerable fuel savings in hot water heating systems, while generally assuring a supply of water at adequate temperatures to all units, the systems do not accurately account for changes in heat loss with changes in ambient temperature. If the ambient temperature is 90.degree. F., there will be a small heat loss along the pipeline, so that a lower than usual temperature T.sub.1 is sufficient at the water tank outlet. On the other hand, if the ambient temperature is 20.degree. F., there will be considerably greater heat losses along the pipeline, and a higher T.sub.1 is needed to assure an adequate water temperature at all units. Attempting to determine heat losses along the pipeline by determining the amount of fuel used when there is minimal demand, is inadequate, especially for larger units where there may always be some demand, and because the amount of heating may be difficult to judge where the pressure of gaseous fuel varies. A hot water heating system which accounted for changes in heat losses along the pipeline to vary the desired temperature at the water heater outlet, so as to assure an adequate but not excessive hot water temperature at the last unit along the pipeline, would be of considerable value.